ORIENTATIONAL DISORDER, THE ORIENTATIONAL DENSITY DISTRIBUTION AND THE ROTATIONAL POTENTIAL IN C-60

Bragg intensities from neutron and X-ray diffraction data of C-60 Sing le crystals were used to determine the nuclear- and electron-density d istributions of C-60 at room temperature. The anisotropic density dist ribution is reconstructed by the maximum-entropy method and evaluated in terms of symmetry-adapted spherical harmonics. From this analysis, the orientational probability density function f(omega) has been calcu lated and the rotational potential V(omega) that is experienced by a C -60 molecule in the cubic surrounding at 295 K has been obtained. f(om ega) shows strong deviations from the uniform orientational probabilit y density function that would result from isotropic rotation. Accordin gly, V(omega) exhibits well developed minima. The absolute potential m inimum is found at an Euler angle set omega(1) and a second set of min ima at slightly higher energy at omega(2). The potential difference be tween V(omega(1)) and V(omega(2)) is 313 K, whereas the overall rotati onal potential barrier height amounts to 522 K. omega(1) and omega(2) are comparable with the major and minor orientations that are adopted by the molecules in their low-temperature arrangement. The angles omeg a(1) and omega(2) are fixed by the intrinsic geometry of the Euler-ang le space (alpha, beta, gamma) under the combined action of the cubic s ite and the icosahedral molecular point group.